The invention relates to a method for operating a sensor, in particular a NOX sensor, for determining exhaust gas components, in particular for a motor vehicle, and a device for operating such a sensor.
Alongside solid particles, nitrogen oxides belong to the limited exhaust gas components which are generated during combustion processes and the permitted emissions of which are being continuously reduced. A variety of methods are currently used in order to minimize these exhaust gas components in the case of internal combustion engines operated in motor vehicles. Reducing nitrogen oxides can be carried out by means of catalytic systems, with reducing agents additionally being used in oxygen-rich exhaust gas. These methods are known under the collective term SCR methods, with SCR denoting “selective catalytic reduction”. A detailed explanation of such methods can be found in DE 34 28 232 A1.
In practical applications, ammonia or ammonia-eliminating compounds, such as urea or ammonium formate, in a solid or solution form are used as reducing agents.
In order to be able to carry out an effective catalytic reduction, for example, with the selective addition of urea, it is necessary to perform a determination of NOX emissions by means of NOX sensors. A method for controlling an exhaust gas after-treatment system is described in DE 101 00 420 A1, in which a predefinable quantity of reducing agent is supplied as a function of the status of the internal combustion engine and/or of the exhaust gas after-treatment system.
The NOX sensors used are very sensitive to alkali and alkaline earth metals by which they are irreversibly damaged. Alkali and alkaline earth metals enter into the exhaust gas via the fuel and the engine oil and from there come into contact with the NOX sensors which are arranged in the exhaust gas system. Particularly when using heavy oil, as is used in the case of large ship engines, large quantities of metal enter into the exhaust gas system which lead to damage of the NOX sensors installed there.
US 2005/0262833 A1 discloses a method for operating a NOX sensor in which the NOX sensor is only at times exposed to the exhaust gas flow in a bypass line of an exhaust gas channel. A connection to the ambient air is established in measurement pauses.